One of conventional technologies extracts edge points in traffic lane lines on a road from an image obtained from a camera installed to a vehicle, and obtains the coordinates of each of the edge points.
Then, the conventional technology inputs the obtained coordinates of each of the edge points as observations of a corresponding one of variables to a previously prepared filter, such as a Kalman filter. Then, the filter estimates state values of a road, such as values of road parameters, which include, for example, a lateral offset position, a yaw angle, and a curvature of the road relative to the travelling direction of the vehicle, i.e. the optical axis of the camera.
The filter has a controllable response to the observations input thereto. For example, the filter sets a level of the response to a higher level upon the vehicle travelling or is about to travel on a curved road to thereby prevent response lag to the input of the observations. In contrast, the filter sets a level of the response to a lower level upon the vehicle travelling on a straight road to thereby prevent the vehicle from zig-zaging.
Japanese patent application publication No. 2006-285493, which will be referred to as a published patent document, discloses the following technology. Specifically, the disclosed technology changes the response of a filter, which is installed in a vehicle, depending on the curvature of the road on which the vehicle is travelling; the curvature is estimated based on a yaw rate of the vehicle observed as an observation.